1. Field of the Invention
The present invention relates to a method of forming a thin film on a substrate and an apparatus therefor, and more particularly to improvement of a method of forming a thin film by a CVD (Chemical Vapor Deposition) method and an apparatus therefor.
2. Description of the Prior Art
In recent years, since manufacturing process for electronic devices including LSIs (large scale integrated circuits) and liquid crystal displays are effected at low temperature and low damage, attention is drawn to the afterglow microwave plasma CVD method of forming a thin film on a substrate at low-temperature conditions. The afterglow microwave plasma CVD method is technology where a reaction gas is excited by microwave plasma discharge in a plasma discharge chamber separated from a reaction chamber, and active species thus produced are and deposited onto a substrate heated to low temperature in the reaction chamber whereby a thin film is formed. In this method, since the substrate is not exposed directly to the plasma, charged particles in the plasma do not damage the substrate or the thin film being formed, and the thin film can be formed at the substrate temperature as low as 300.degree. C. or less.
FIG. 1 shows a sectional view of a thin film forming apparatus to be used in the afterglow microwave plasma CVD method according to the prior art, for example, as disclosed in Japanese patent application laid-open No. 27656/1983.
In FIG. 1, numeral 1 designates a reaction chamber, 2 a first reaction gas supply port, 3 a first reaction gas, 4 a second reaction gas, 5 a microwave plasma discharge tube, 6 microwave energy, 7 an active species supply port, 8 active species, 9 a substrate, 10 a heater on which the substrate 9 is held, 11 a gas exhaust port, and 12 an exhaust gas.
Next, the operation will be described. In the thin film forming apparatus constituted as above described, for example, when a silicon oxide film is formed, a silane gas 3 as the first reaction gas is supplied from the reaction gas supply port 2 into the reaction chamber 1, and gas 4 including oxygen atoms as the second reaction gas is supplied to the microwave plasma discharge tube 5 the activated oxygen atoms 8 thus produced are supplied from the active species supply port 7 into the reaction chamber 1.
Then the activated oxygen atoms 8 produce vapor phase chemical reaction with the silane gas 3 within a space in the vicinity of the substrate, and form a precursor gas which contains silicon atoms, hydrogen atoms and oxygen atoms. The precursor is contacts the surface of the substrate 9 and a silicon oxide film.
FIG. 2 is an enlarged view of a reaction chamber 1 of a thin film forming apparatus to be used in the afterglow microwave plasma CVD method of the prior art. In such a thin film forming apparatus, active species 8 supplied from the active species supply port 7 into the reaction chamber 1 do not stagnate in the vicinity of the surface of substrate 9, but are diffused to an outer circumferential portion of the substrate 9 and further in the vicinity of an inner wall of the reaction chamber 1, where the active species 8 react with first reaction gas molecules 3 diffused in similar manner and forms a precursor 13.
A part of the precursor 13 formed in the vicinity of the surface of the substrate 9 also diffuses to the vicinity of the inner wall of the reaction chamber. The precursor 13 does not take part in the thin film forming reaction, but is contacts with a surface at low temperature, for example, the inner wall of the reaction chamber and forms a reaction product 14 of fine particle form. In the prior art, the reaction product 14 becomes dust and floats in the reaction chamber and may be taken into the thin film or adhere to the surface of the thin film, thereby interferring with the information of a thin film of high quality.
The afterglow microwave plasma CVD method is characterized in that when the precursor 13 is formed by the chemical reaction of the active species 8 with the first reaction gas molecules 3, it is accompanied by chemical luminescence. Since the chemical luminescence intensity has is correlated with the thin film forming rate, the measurement of luminescence intensity through an optical fiber 15 using a spectrophotometer 16 thereby permits in-situ monitoring of the thin film forming rate. In the thin film forming apparatus of the prior art, however, during measuring the chemical luminescence intensity, since the forming reaction of the precursor 13 is not limited a position on the surface of the substrate 9 as shown in FIG. 2, in addition to the necessary measurement of the integrated intensity of the chemical luminescence within the substrate diameter Lw the luminescence intensity outside the substrate (Lc-Lw) not taking part in the thin film forming reaction is also measured, thereby making it difficult to perform the in-situ monitoring of the thin film forming rate at high precision.